Transmission of useful signals, i.e., data such as binary-coded data, is usually performed by frequency modulation of a carrier signal by frequency shift keying (FSK). FIG. 1 schematically illustrates a standard transmitter for FSK modulation of a carrier frequency. The transmitter has a direct digital frequency synthesizer (DDFS) to perform the real part (in-phase, I) and the imaginary part (quadrature phase, Q) for a complex up-conversion of the carrier signal by means of a phase-locked loop circuit (PLL). The PLL circuit has a frequency divider which generates an output frequency from a reference frequency fref (the output frequency being at a frequency higher by a factor N). Using a fractional-N synthesizer (FNS), the output frequency may also be above the reference frequency by a non-integral factor N. The phase-locked loop PLL also has a voltage-controlled high-frequency oscillator (VCO), which is regulated at a stable accurate reference phase of the reference frequency fref by means of a phase comparator. The reference frequency fref is supplied (for example) by a quartz oscillator (see top of FIG. 1).
As shown in FIG. 1, the output signal of the PLL is output firstly without a phase shift (real part, in-phase, I) and secondly with a 90° phase shift. The real part and the imaginary part (quadrature phase, Q) of the carrier signal are then each modulated separately (IQ modulation) and are next added to the output signal of the direct digital frequency synthesizer.
Such a circuit permits accurate and stable FSK modulation, but it has disadvantages. It requires two fast digital-analog converters (DAC) and two high-frequency (HF) mixers, and the circuit occupies substantial area, including the required tables for the sine and cosine transformation. The two digital-analog converters and the two HF mixers consume a significant portion of the total energy for operation of the transmitter, often between 15% and 20%. Furthermore, the direct digital frequency synthesizer (DDFS) also generates noise and interference signals. The circuit branches for the real part I and the imaginary part Q must be coordinated well with regard to phase and amplitude in order to suppress minor frequency signals. These last effects make it more difficult to comply with the requirements of regulatory authorities with regard to unwanted emissions by the transmitter.
Modulation by direct frequency shift keying (DFSK or direct FSK) is achieved by modifying a channel control signal of the fractional-N synthesizer (FNS), i.e., adding the modulation information directly to the basic channel, so that a binary pulse train of the useful signal to be transmitted modulates the high-frequency carrier signal directly, i.e., without an intermediate carrier. FIG. 2 illustrates a corresponding circuit.
Although this circuit avoids the problems with IQ modulation, it has its own problems. The transient response of the fractional N-synthesizer FNS interferes with the modulation pattern in a manner that cannot be reversed easily or analytically. Analog components such as the loop filter and the voltage-controlled oscillator (VCO) may not be stable over time, over variable temperatures, and/or over different production batches, and therefore also have a negative effect on the modulation pattern. The choice of FNS loop parameters is limited because a PLL loop bandwidth far below the data rate causes a Gaussian minimum shift keying (GMSK) modulation characteristic (though that may be desirable in some applications).
The basic principles for solving these problems include a large PLL bandwidth to minimize the effect of PLL dynamics; a fixed modulation gain, which inverts the calculated or estimated PLL transmission function; and a voltage-controlled oscillator (VCO) feedback to regulate the modulation gain in the manner of a closed control loop.
A large bandwidth of the functional N-synthesizer FNS, one beyond the bandwidth required for a sufficiently rapid transient response, necessitates expensive and difficult measures to maintain a low level of FNS noise and of interference signals. A fixed gain obviously cannot have any variations over time, temperature and production batches. VCO feedback fundamentally requires a type of “receiver” and demodulation circuit that includes, according to known approaches, FM demodulators which do not operate under all circumstances, and/or analog components, and/or estimates of a transmission function.